Oil cracking process



April 6, 1937.

W. A. S. HARMON OIL CRACKING PROCESS Filed May 2l, 1934 3 Sheets-Sheet l April 6, 1937.

w. A. s. HARMON v 2,075,896

OIL CACKING PROCESS Filed May 21, 1954 5 sheets-sheet 2 5f l'vewfor. Y

Hayman/. 5 .Far/noia,

fifa/"Hey April 6, 1937.

W. A. s. HARMON- 2,075,896 oIL CRACKING PROCESSY V Filed May 2l,4 1934 5 Sheets-Sheet 3 .Differ/iq? Y or other light products.

Patented Apr. 6, 1937 UNITED STATES PATENT OFFICE 5 Claims.

This invention relates generally to the rening of petroleum oils, and deals particularly with an improved liquid phase oil cracking system for producing from comparatively heavy oils, gasoline The broad objects of the invention correspond to those generally sought, but usually accomplished to a limited extent only, in the operation of systems of this character; for example, to obtain maximum available yields of gasoline from a given charging stock, to reduce to a minimum the fixed gas production and carbon deposits on the heating surfaces, and to obtain highest operating eiiiciency and economy in the system as a whole. 'Ihe invention also involves a great many specic objects, which, in general, are directed to particular means for accomplishing thebroadobjects. A proper understanding of all these can perhaps best be given by first stating certain principles indispensable to the accomplishment of the de` system. f s

In all liquid phase cracking operations wherei the oil is heated in vessels or tubes to cracking temperatures, high proportionate conversion to light fractions or cuts, reduction of fixed gas formation and carbon deposits on the heating surfaces, speed of conversion and production of pressure distillate, and general efficiency, are dependent upon certain considerations. These include the nature of the charging stock, temperatures and pressures in the system, heat transfer rates from the heating medium to the vessel or tube and from the latter to the oil, disposition or treatment of vapors after initial cracking, control of carbon formation and removal of carbon from the oil heating surfaces, and flexibility and controllability of the system as a whole. It may be well to observe how these considerations are met in the most prevalent cracking systems, the tube-and-tank type, in which the oil is heated under high pressure to cracking temperatures during passage through externally red tubes, and then maintained in a pressure reaction chamber over a time period suiiicient to allow the cracking reaction to complete itself as far as possible within practical limits.

There is of course a maximum potential cracked gasoline yield obtainable from any given charging stock, and the closeness of approach to that maximum will depend upon the eciency of the individual cracking system. Considering the matter of heat transfer rates from the heating medium to the oil, the tube-and-tanl; type cracking unit involves first the transfer of heat from hot combustion gases to the cracking tube Wall, and it is well understood that the unit heat transfer rates from gas to metal are low. In the usual cracking tube banks, the gas to metal heat transfer rates ordinarily lie between 3.0 and 4.5 B. t. u./sq. ft./deg. F./hr. in the convection section, and between 7.5 and 13.5 B. t. u./sq. ft./deg. F./hr. in the. radiant section. At the oil sideof the tubes, the unit heat transfer rates are higher, ranging from to 150 B. t. u./sq. ft./deg. TEK/hr., although the latter is exceptionally high and can be maintained only during short time tests.

The oil is pumped through the tubes (still speaking of the usual tube-and-tank type unit) 15 and is maintained under pressure sufficient to partially suppress vaporization of the low lboiling fractions formed in the cracking tubes. However, extremely high pressures, and pressures greater than are utilized in any of the present commercial systems, would be required to hold gasoline fractions in the liquid phase. Consequently, the cracking tubes contain and are completely lled with a mixture of oil and vapor flowing in a turbulent stream in which neither the liquid nor vapor body` is confined to any predetermined zone. While fairly high conversion ratios are obtainable in cracking tubes as described, particularly when aided by recirculation of residue, conditions in the tubes not only present a close approach to the theoretical maximum conversion, but actually cause kunnecessary recracking of a substantial portion of the cracked gasoline vapors, with resultant production of excessive fixed gases and carbon.

These conditions arise largely as a result of the fact that some of the vapors, when generated at the wall of the tube, form a constantly present oil enveloping gas film held under cracking pressure between the tubev and the oil and through which any cracking heat transfer must occur, and also due to the fact-that the cracked vapors, once formed, must remain held inthe hot oil stream for several minutes and throughout the remaining length of the cracking tubes, instead of being immediately released to a vapor zone maintained below the cracking temperature. There results a comparatively low heat transfer rate from tube to oil, and recracking of some of the vaporizedA cracked gasoline fractions due to the vapors being continuously held under high pressures and temperatures and in contact with the tube wall solution. Concomitantly, fixed gas forms as the gasoline fractions recrack, and heavy carbon deposits form on the tube walls. The .faults vapor space. By virtue of certain conditions the oil lm stream hereinafter explained in detail,

there is established, instead of the usuall'coriiparatively low metal-to-oil heat transfer rate` seldom even approaching 150 B. t. u./s'c1. ft/de'g'. F./hr., l a boiling heat transfer rate, heretofore unattainable to my knowledge in oil cracking processes o f Addiat least 2000 B. t. u./sq. ft./deg. F./hr. tionally, and also by virtue of the lmed condition of the oil, almost instantly upon their formation at the tube surface, the cracked gasoline vapors are released through the ilm into a subcracking temperature vapor zone to prevent recracking of the vapors. In rapidly removing the vapors from a high temperature cracking to a low temperature subcracking zone, recracking and undue fixed .gas formation is prevented. The carbon that necessarily forms duringl the cracking reaction, is

promptly and continuously picked up and removed f in the oil nlm stream. While confined to a deined zone not occupied by the oil, the cracked vapors released through the oil lmare maintained in: contact with thev oil in being caused to flow throughthe tube.V counter-current tothe oil `heat the tubes by' circulating ahigh heat conduc- 703' the methodas denned in the claims.

lm stream. Y

In order to obtain higher heat transfer rates at the outside surface of the tubes and also to facilitate more exact temperature control, I preferably tive` molten vmetal or salt in contact with them. And while I prefer, for,various reasons, to `use lead as the-y tube heating medium, it ywill beunfderstood that conditions permitting, other metals `or salts may be used instead.\ By thus exteriorly heating the. tubes 'with` molten lead; I obtain: in'-` steadv of the usual low gas-to-metalheat transfer ratesy seldom exceeding 14 B. t. u./sq. ft./deg. F./hr., a metal-to-metal transfer rate, under average conditions, of around 4000 B. t. u./sq. ft./deg. 15E/hr. i Y

The inventionv isV characterized by the fact that temperature, pressureand conditions in the oil stream which go to influence the cracking operation arescapa-ble of close control, the `outside ,tube temperature may be regulated with exactness, and

the nlm. velocity and thickness controlled within` close limits by regulating thev vapor density or pressure and the rate of vapor flow within. and counter-current tothe oil Vfilm stream.

While thus far and for convenience of discussion I have referred to certain general aspects of the invention as applied tov apparatus in which.

the oil is passed in a film stream Within an exteriorly heated tube, it will be understoodv that in its broad aspects, theinventionzis not limited to the use of cracking tubes or any other particular form of cracking apparatus, but applies` broadly to any form of apparatus for carrying out Preferably, however, I prefer to utili-ze as the cracking unit proper, an oil lm heater comprising one ormore vertically extending tubes.

The above mentioned and` Various. additional Fig.,4.is a section on line 4 4 of Fig. 3.

Fig. 5' is'a flowdiagram of the lead circulating 'sys'tem;land

Fig. V6 is a diagrammatic View illustrating the probableY conditions existing in the oil films being subjected to cracking and vaporizaticn in the cracking unit.

The invention may be most clearly described byrst following through the now sheet of Fig. 1, and then by taking up specifically and more in detail certain parts of the system with which the inventionis more particularly concerned. The characteristics of the charging stock supplied from tank I0, may be predetermined in accordance with the products desired, and operating conditions within the system. Any water contained in the charging stock may be preliminarily freed by first forcing the charging stock by pump I I v-ia line I2 through vapor cooler I3, line I4a and the tubes in the convection section 23 of preheater retort I5, and' thence through line I5 into water vapor separator I'I. The latter consists of a tank havinga constant liquid level float control I8, and -vent line I9, the flow of water vapor through which is controlled by a suitable floatoperated valve 20. The charging stock is-,then passed under pressure by pump- 2| through line 22, convection and radiant tube sections 23 and 24 of retort I5, and thence via line 25 into the top of dephlegmator 26. In the event the charging stock is substantially dry, water vapor separator. I'I may of course be dispensed with.

Retort I5 is heated by hot waste gases from the lead retort, hereinafter described, and introduced through passage 21, any additional heat required to preheat thev oil to desired temperatures being supplied by burner 28 which may be controlledby a suitable regulator, not shown, in accordance` with the outlet temperature of the oil leaving the radiant tube section 24. The waste gases fromthe oil preheater retort pass through an air preheater 2U counter-current to an air streamto be used as combustion air in both the eiland leadretorts, and forced through the system by fan 30. Passage 3I, receiving the preheated air from heater 29, communicates with passages 32 and 33 leading to the oil preheater and lead retort burners.

The preheated charging stock in line 25 enters the dephlegmator through a valve 34 controlled by float operated regulator 35 acting to maintain the oil in the lower section of the dephlegmator at a constant predetermined level. The oil introduced into the top of the dephlegmator passes downwardly over trays 26a, or other suitable liquid-vapor contact media, in thorough admixture and intimate contact with vapors being discharged from line 36 into the dephlegmator ata point below trays 26a. As will later appear, these vapors consist mainly of cracked and the lighter product fractions, forv example gasoline and kerosene, although they will also carry substantial quantities of heavier fractions extending perhaps through the gas oil range. The charging stock reflux in the dephlegmator serves to condense and absorb all such heavier fractions as it may be desired to return to the cracking cycle, and also to retain in the system the latent heat of the heavier fractions given up through condensation.

Under some circumstances, depending upon temperatures and other operating conditions in the system, if all the charging stock were to be passed down through the dephlegmator, the resulting temperature of the oil in the base thereof may become excessively highand may reach the point at which undesirablepreliminary; cracking Will occur. In order to anticipatethis possibility. I provide a by-pass line 38 leading from a point in advance of valve 34 into the base of the dephlegmator, and carrying a valve 39 which is controlled in accordance with the temperature of the oil body in the bottom of the dephlegmator. Valve 39 permits a sufficient portion of the comparatively cooler charging stock to by-pass directly into the base of the dephlegmator, and to keep the temperature of the oil body therein from exceeding a predetermined maximum. In the event the remaining portion of the charging stock stream is nsuncient to provide Vthe necessary amount of reflux for the dephlegmator, make-up reflux may be supplied by returning to the dephlegmator some of the nal condensate through line 40.

From dephlegmator 26, the'preheated charging stock passes through line 4I to pump 42 and is forced by the latter through line 43 to the top of the oil film heater or cracking unit generally indicated at 44. 'Ihe latter is hereinafter described in detail, and it will suffice to observe at this point that the charging stock is passed downwardly through the oil film heater and subjected to cracking and vaporization, the'vapors being released through line 36 to the dephlegmator, and the unvaporized residuum being discharged through line 45 to separating cylinder 46, the residuum being introduced tangentially into the latter at some suitable point below the oil level L. The residuum is maintained at a constant level L by means of a float controlled regulator 4l operatively connected to valve 48 in the feed line 4l. Line 4I connects with the separating cylinder via line 4|a below the point at which line 45 discharges into the cylinder but at a subtantial distance above its base. Assuming pump 42 to operate at a constant capacity rate, regulator 41 and valve 48 act to maintain a predetermined oil level in the separating chamber by increasing or decreasing the pump supply through line 4| as the flow of residuum from the separator to the pump respectively decreases or increases.

Pump 42 thus normally feeds to the oil lm heater, a mixture comprising preheated charging stock and residuum from separating cylinder 46. Cylinder 46 is provided primarily for the purpose 0f allowing carbon or other solid bodies to settle and separate out of the oil, and in general practice, the separating cylinder need not have a specially large capacity with the View to maintaining the residuum under pressure and at substantially the temperature at which it leaves the oil lm heater, for any long period of time. Cylinder 46 is primarily a carbon and oil separator and need only be sufficiently large to hold the residuum long enough (say ve minutes) to permit the necessary proportion of the suspended carbon to settle out. Carbon settling to the base of the separating cylinder may be withdrawn along with some of the residuum, through line 5i? and passed through cooler 5I to storage Aor a coking still.

, Insofar as disposition of the residuum in the separating cylinder is concerned, the system may operatel in several different manners. For example, it may be desired to crack the charging stock to produce a high gasoline yield and still produce a residuum conforming to free carbon specifications for fuel oil. Under such conditions, pump 42 will continuously circulate a mix.- ture of charging stock and residuum from the separator through the cracking zone in the oil lrn heater 44. The cracked" gasoline vapors formed per pass of charging stock and residuum through the cracking Zone may, for example, be within 10 to 15% of the feed, depending upon the characteristics of the charging stock, and depending upon the permissibler percentage of free and returned to the cracking cycle, the recircu-A lated resdiuum being drawn from the separator at a point between the liquid level and the residuum outlet at the base. In the oil body below outlet 4Ia, suspended carbon particles settle out and accumulate until the residuum 4draw-off through line 50 will meet required specifications. Assuming a constant proportionate rate ofy feed of charging stock and residuum 'to'. the oil film heater, the desired fuel oil residuum specification can be met by changing the pressure in the` system, by varying the cracking temperature in the oil lm heater, or by a combination of both.

, Assuming it to be desired to operate the system on a non-residuum cycle, that is one in which the end products are gasoline and coke, either of two methods may be employed: Instead of recirculat ing residuum to the oil film heater, all the residuum may be withdrawn from the separator, taken to a separate coking still (not shown), and run to coke, the vapors from the coking still being l separately condensed or returned to the cracking cycle with the fresh feed. As an alternative, all

the residuum within the separator may be recirculated through the oil lm heater and the carbon and coke permitted to accumulate in the bottom of the separating cylinder, to be withdrawn continuously or intermittently lremoved byfsuitable means.

The oil film heater, detailed in Figs. 2 to 4, comprises a vertically extending shell 53 containing a plurality of open oil cracking tubes 54 whose lower ends extend through tube sheet 53al and whose upper ends extend through the top closure 53h of the shell and tube sheet 55 formingv the bottom of an oil distributing head 56. Heating fluid, preferably molten lead, is fed* into the bottom of shell 53 through inlet 51 and is discharged from the upper end of the shell through outlet 58. In addition to fiowing in a general path longitudinally of the tubes, and countercurrent to the down flowing oil film streams therein, the molten lead is also caused to flow transversely of the tubes and in reverse directions by a system of baflles 59. Except for its lower portion 53e which connects with line 45 leading to the separating cylinder, shell 53 is encased within the lead retort combustion gas passage 68 and is thus exteriorly 5 heated.

The oil distributer head 56 comprises a shell 6l having a feed oil 62 connecting with line 43 and discharging into a tray 63 positioned directly above oil filming nipples 64, see Fig. 3, attached lo to the upper ends of cracking tubes 54. The oil fed into tray 63 drains through apertures 65 into the base Gla of the dstributer head, and thence fiows through a plurality of non-radial slots 66, see Fig. 4, in the oil filming nipples. By virtue l5 of the arrangement of these slots, the oil passing through them is given a swirling motion and is caused to fiow spirally in a film-like stream down the inside surfaces of the cracking tubes. Giving the oil film stream a spiral motion within the 2O tubes increases the heat transfer rates between the tubes and oil film, the time of exposure of the oi-l to cracking temperatures, and the effectiveness of the oil stream in removing carbon particles from the tube wall.

Sections 54a of the tubes directly belowr the distributer head are not heated for the reason that it is desired first to cause the oil film to be established and the inner tube surface to be completely wetted before the oil enters the cracking zone, that is, the lower portions of the tubes within shell 53 contacted by the molten lead. Additlonally, it is desired to avoid contact between the molten lead and tube sheet 55 so as not to crack the oil and deposit carbon within the distributer head.

A fragmentary section of one of the cracking tubes and the probable conditions within the oil lm adjacent the wall of the tube are diagrammatically shown in Fig. 6. The oil film 68, surrounding a central vapor space 69 within the tube, may be regarded as comprising sections or layers 68a, 68h and 88e, in each of which the oil molecules are subjected to different and distinctly important conditions. In layer 68a contacting the wall of the tube, the downward velocity of the oil molecules is substantially zero, and the heat transfer through this layer is by conduction and also by convection, depending upon the molecule vibration rate induced by heating to high temperatures and vaporizing the oil molecules. In layer 68h, the oil flows in a downwardly spiralling path, and in layer 68o, at the oilvapor interface, the oil molecules are given a turbulent, upwardly turning motion, as shown by the arrows, due to the upward velocity of the oil vapors in space 69. The arrows at 10 indicate the probable path of a reformed low boiling point molecule resulting from cracking and vaporization Within the tube contacting layer 68a.

The oil molecules within layers 68a quickly approach the temperature of the tube Wall and when that temperature is above 950 F., cracking of the heavier oil molecules and formation of low boiling point molecules occurs almost instantaneously. As a result of their instantaneous vaporization, the reformed molecules create turbulence within layer 68a, lower the resistance of the layer to heat flow, and set up a boiling condition at the metal surface of the tube Wall. Thus 70 instead of the usual metal-to-oil heat transfer rate, I obtain a boiling heat transfer rate many times greater.

In order to obtain this boiling heat transfer rate, it is essential that there be space available into which the' molecule vaporizing at the metal surface can immediately escape, and that as little resistance as possible be offered to escape of the vaporized molecule into the vapor space. In the present instance, the vaporized molecules have only to traverse the oil film 68 in order to reach the vapor space 69, and it will be readily apparent that .the oil lm will offer no appreciable resistance to escape of the vaporized molecules.l As the resistance to vapor escape from the heating surface decreases, the rapidity of vapor release and turbulence within the oil film increases, as does also the conduction of heat throughout the film. It is important to observe that instantly upon their formation, the cracked gasoline molecules pass into thelower temperature vapor Zone 69, thus rendering it impossible for these molecules to become reheated and recracked with resultant formation of waste fixed gas and carbon.

The rate and condition of oil fiow through the tubes is such that the inner surfaces are wetted throughout their lengthsythereby establishing a necessary condition for obtaining boiling heat transfer rates. Carbon released in the molecular reformation is picked up and washed from the tube surface by the oil film stream and carried to the separating chamber by the residual oil leaving the bottoms of the tubes.

The oil film and vapor within each tube flow counter-currently, with resultant heat transfer at the oil-vapor interface. Due to the motion of the oil particles in film layer 68C, there results an intimate admixture of such particles with those in the adjacent layer 68h, and a transfer of heat from 68e to 68h such as to rapidly elevate the oil film temperature throughout. Thus the oil film receives heat from both the oil-metal and oil-Vapor interfaces while the high temperature cracked vapors are being projected through the lm. By reason of the oil film and vapor stream counter flow, a certain amount of recycling or rectification occurs within the tubes themselves, some of the heavy vapor fractions being condensed through contact with the colder oil, particularly in the upper tube portions.

As previously stated, the vapors leave the oil film heater through line 36 and the heavier ends are removed in dephlegmator 26. The uncondensed vapors, mainly gasoline and kerosene, then flow through line 13 and coolers I3 and I4, and the condensate and uncondensed gases through lines l5 and 16 to gas trap 11. From the latter, the fixed gases are released through fioat controlled valve 18 in line 19, and the pressure distillate through line 8U. Any suitable back pressure (the cracking pressure) may be maintained in the system by adjusting valve 8| in the pressure distillate discharge line 80. Referring again to Fig. 1, a vapor line 83 leading from separator 46 connects with a vapor by-pass line 84 connecting with the base 53e of the oil film heater shell, and with outlet vapor line 36. Depending upon desired operating conditions within the oil film heater, all or any selected portion of the vapors released from the residuum in the separator may be returned through lines 83 and 84 to the base of shell 53, circulated up through the cracking tubes and combined with the vapors being formed therein. The proportion of the vapors circulated from the separator to the oil lm heater and those passed directly to vapor line 36, may be controlled by valve 86 in line 84.

The downward velocity of the oil film streams in the tubes is controllable by regulating the upward velocity of the contacting vapors, and therefore the resistance to fiow of the oil streams. 'Ihe Cil aes-,ste

vapor velocity, in turn, is' controllable either by adjusting bypass valve 86, by'adjusting the final pressure control'valve" 8| tov vary the specific v; por volume, or by a combination'nof both.

5 Through ability toregulate the oil lllm stream velocity, I am able to control the time of `oil :conitact with the heating surfaces. By prolonging the time oi contact, the maximum yield of distil- Y late may be obtained by one pass ofthe charging 10 stock through the tubes,` o r byl shortening the time of contact 'and repeatedly recirculating the unvaporized -residuum, Awith make-up 'charging stock, through the tubes. 'A f The lead heating and circulating `system coml 5 prises a retort 88 having a combustion* chamber 89 and passages 90 and 9| which-conduct 'a--po'r tion of the hot gases into passage 92 and in con-v tact with the oil film heater shell 53. Thecombustion gas ow past heater shell 53 is controlled 2O by a damper 93 which in turn may be regulated by any suitable means in accordance with the outlet lead temperature in line 94 connecting with the oil lm heater lead outlet 58, see Fig. v2. Molten lead is circulated by a pump 95 via line 25 ,Q6 through the retort heating coils 91 in which the lead is heated to a temperature of from 1000 to 1200" F., or above, depending upon the characteristics of the charging stock and operating conditions. The molten lead then flows through 30 line 98 and inlet 57 into the lower portion of the oil film heater shell, passes upwardly in contact with the cracking tubes and discharges through outlet line 9@ into a drum 99 located within combustion gas passage tl. Pump 95 takes suction 35 from the lead receiving drum S9 through line |00. It will be noted that all the lead circulating lines are contained within the hot combustion gas passages of the retort, thereby minimizing heat losses from the lead stream and providing effec- 40 tively for its maintenance at the necessary high temperatures.

While it will not be necessary to describe pump 95 and the lead circulating system in full detail, as these matters are more particularly treated 45 in my copending application on Molten metal pump, Ser. No. 726,652, led on even date herewith, for the sake of completeness in describing the present system as a whole, I will refer briefly to certain aspects of the lead circulating system 50 and its general mode of operation.

Referring now to Fig. 5, which diagrammatically illustrates the lead circulating system, all the lead for the system is initially charged into drum 99. After the lead has become molten,

55 inert gas supplied under suitable pressure by pump lS through line |09, is admitted to the drum above the lead level through the line and under control of pressure regulator valve |02. The gas pressure in drum 99 is increased until the molten lead has become elevated through line |00 into the lower portion of the pump 95 and through pipe |0211, into float chamber |021) to a predetermined level |03. At the same time, the molten lead lls the retort heating coil 01 G and rises within the lower portion of shell 53 to a corresponding level. Valve |04 is then opened to admit inert gas through line |05 into gas chamber |06 of the float reservoir and under control of float operated valve |01. The inert 7o gas admitted to the iioat chamber and through pipe ||0 into the barrel of pump 95, prevents the molten lead from rising above level |03 and into the upper bearing ||I of pump shaft H2 being driven by motor H3. Additional gas is 75 admitted through valve |02 to drum 99 to elevate the leady through line 98 to the top of the oil film heaterand to completely ll the latter above level |03. Byl opening vent valve ||5in line I.| 6, gas or air may be expelled from thev system as the latter lls with the molten lead. The valve controlled ventalso serves as a means for determining when thelead has reached the high point in the system. After the entire lead circulating system is filled with molten lead, valve ||5 is closed VVand circulating pump 95`started into .operation. In shutting down .the lead circuit, it is only necessary'to stop the pump, release the inert gas pressure and allow all the lead to drain back into drum 99.l i

I claim:4 H '.l. The method that includes, passing a lmlike stream composed exclusively of oil cracking stock, downwardly over a vertically extending heating wall, said film-like stream -being of uniform thickness excepting las its thickness Ais aiected by diminution of the oil4 by vaporization, heating said wall to a temperature suflicient to crack, and vaporizeA oil in that portion of the stream immediately adjacent one surface of,said f wall, uniformly releasing the oil vapors directly through the stream from the points at which said vapors are formed, into a vapor zone adjacent the exposed surface thereof, passing the vapors in a path counter-current to said oil stream, and regulating the rate of oil flow in said stream by varying the counter-current vapor velocity by controllably imposing a variable back pressure on said vapors. y.

2. The method of converting hydrocarbon oils comprising passing the oil downwardly over the inner surface of an internally unobstructed vertically oriented and substantially cylindrical heating tube of small diameter as compared with its length, in a hlm-like stream of progressively increasing temperature in its direction of flow, thereby leaving an unobstructed central vapor space surrounded by said nlm-like stream, circulating a stream of molten metal in contact with the outer surface of said tube and thereby heating the outer surface ofthe tube from the outside only, to a temperature causing oil in that portion of the stream immediately adjacent said inner surface of the tube to crack and liberate cracked vapors into said central vapor space, and subjecting vapors to condensation and recycling within the tube by passing them upwardly through said space in direct contact with the surrounding and relatively cooler portion of the oil-lm stream in the upper portion of said tube.

3. The method of converting hydrocarbon oils comprising passing the oil downwardly over the inner surface of an internally unobstructed vertically oriented and substantially cylindrical heating tube of small diameter as compared with its length, in a film-like stream of progressively increasing temperature in its direction of ow, thereby leaving an unobstructed central vapor space surrounded by said iilm-like stream, circulating a stream of molten metal in contact with the outer surface of said tube and thereby heating the outer surface of the tube from the outside only, to a temperature causing oil in that portion of the stream immediately adjacent said inner surface of the tube to crack and liberate cracked vapors intov said central vapor space, subjecting vapors to condensation and recycling within the tube by passing them upwardly through said space in direct contact with the surrounding and relatively cooler portion of the oil-film stream in the upper portion of said tube, vwithdrawing unvaporized residuum from .said tube, and returning at least a portion of said residuum to the upper portion of said oil stream. 4. The method of converting hydrocarbon oils comprising passing the oil downwardly over the inner surface of an internally unobstructed vertically oriented and substantially cylindrical heating tube of small diameter as compared 10 with its length, in a nlm-like stream of progressively increasing temperature in `its direction of flow, thereby leaving an unobstructed central vapor space surrounded by said nlm-like stream, heating the outer surface of said tube from the outside only, to a temperature causing oil in that portion of the stream immediately adjacent said inner surface of the tube to crack and liberate cracked vapors into said central vapor space, subjecting vapors to condensation and recycling within the tube by passing them upwardly through said space in direct contact with the surrounding and relatively cooler portion of the oil-iilm stream in the upper portion of said tube, and regulating the rate of oil flow in said stream by varying the counter-current vapor velocity.

5. The method of converting hydrocarbon oils comprising passing the oil downwardly over the inner surface of an internally unobstructed vertically oriented and substantially cylindrical heating tube of small diameter as compared with its length, in a nlm-like stream 4of progressively increasing temperature in its direction of flow, thereby leaving an unobstructed central vapor space surrounded by said lm-like stream, heating the outer surface of said tube from the outside only, to a temperature causing oil in that portion of .the stream immediately adjacent said inner surface of the tube to crack and liberate cracked vapors into said central vapor space, subjecting vapors to condensation and recycling within the tube by passing them upwardly through said space in direct contact with the surrounding and relatively cooler portion of the oil-'illm ystream in the upper portion of said tube, and regulating the rate of oil iiow in said stream by varying the counter-current vapor velocity by controllably imposing a. variable back pressure on said vapors.

WAYNE A. S. HARMON. 

